An x-ray tube outputs radiation across a wide range of energy bands, the distribution of the energy being defined by the accelerating voltage applied to the tube. When x-rays impact a material, they are absorbed as they pass through. X-rays of different energies are absorbed differently which means that the initial x-ray intensity profile changes. Different materials cause a distinctive change in shape of the x-ray intensity spectra and thus if the spectra can be recorded with sufficient accuracy, it is possible to predict the material that the x-rays have passed through.
While the mass absorption coefficient depends upon both the material type and also the energy of the incident photons, the mass absorption coefficient is independent of material thickness and density. Hence, faced with a resultant spectrum, and knowing the starting spectrum, it is possible to deduce the mass absorption coefficient values and hence the material type the x-rays have passed through.
The detection of x-rays falls into two categories. The first is direct detection, where the energy of an x-ray photon impinging upon a particular material, such as CdTe or Ge is absorbed and converted into an electrical signal. The second is indirect detection in which an intermediate scintillator material first converts x-ray energy into visible light which is subsequently converted into an electrical signal by a detector.
Direct detection has particular application in the identification of materials.
X-ray detectors are typically operated in one of three modes: pulse mode, current mode and voltage mode. Current mode is used in cases where event rates are high and voltage mode is used for high energy detection. Pulse mode operation is widely preferred as it preserves amplitude, counting and timing information for individual pulses.
Direct detection using pulse mode allows materials to be identified and is described in a number of published patent applications. For example:
The international patent application published under number WO2008/142446 describes energy dispersive x-ray absorption spectroscopy in scanning transmission mode involving the calculation of the intensity ratios between successive frequency bands;
The international patent application published under number WO2009/125211 describes an imaging apparatus and method;
The international patent application published under number WO2009/130492 describes the determination of composition liquids; and
The international patent application published under number WO2010/136790 describes a method for the identification of materials in a container.
Whilst the techniques set out in the patent applications mentioned above are effective, the detectors themselves present limitations.
Pulse mode detection provides counting and energy resolution information in the form of an x-ray spectrum. This x-ray spectrum, also referred to as a pulse height spectrum is typically produced by measuring the height of each pulse from the detector. A spectrum of the total number of detected counts per energy range (typically referred to as energy bins) is produced with the width of any given energy bin configured according to limitations such as detector resolution, electronics selection and input count rate.
The pulse mode detection technique has been adopted in many materials identification applications because of the preservation of photon counting and energy information for individual pulses.
A major issue limiting the materials sensitivity of energy dispersive detectors, the ability of the detector to detect different materials, is that these detectors have count rate limitations. Unlike current or voltage mode detectors where the time averaged current or voltage is measured, the electronics used in pulse mode detection must analyse the pulse from each x-ray interaction with the detector. As these pulses have a finite width in the time domain they begin to overlap as the count rate is increased. This phenomenon is known as pulse pile up and distorts the x-ray spectrum.
In cases where samples exhibit large region to region variation in thickness or density it is possible that some detectors in an array (pixels) may see very high count rates while neighbouring pixels may see very low count rates. Pixels directed along the path of low density and/or thin sample path lengths may see rates which are in the extreme pulse pile up regime, leading to distortion of the energy spectrum.
The obvious way of avoiding such pulse pile up problems is to reduce the input count rate by reducing the beam power or increasing the source to detector separation. The problem with a global reduction in x-ray flux is that highly absorbing regions fall into the measurement noise floor and become indistinguishable. Contributions to the measurement noise floor include spurious dark counts and Poisson noise, both of which become significant at low count rates. This makes global changes in x-ray flux undesirable and requires multiple shots to be taken in order to resolve each contrast level. This approach is time consuming and increases the absorbed x-ray dose.
In materials identification applications users often require the shortest possible measurement time. Nowhere is this more important than in security scanning where, for example, high volumes of luggage must be scanned rapidly. This results in short integration times which in turn result in either higher Poisson errors or spectral distortion due to pulse pile up. These distortions in the energy spectrum limit the sensitivity of materials identification techniques, therefore limiting the materials which can be distinguished. Consequently, minimising spectral distortion is at the expense of counting errors and measurement time.
Another way to avoid such pulse pile up is to reduce the width of the pulse produced in the detector electronics thereby minimising the probability of two pulses piling up. This leads to errors in the measurement of the pulse height (and therefore x-ray energy) known as ballistic deficit and the processing of such pulses requires faster analogue to digital sampling, lower noise amplifiers and low capacitance, fast rise time electronics. All of these features add to the cost and complexity of the detector electronics.